The present invention relates to an improvement in refractory plates for apparatuses for the continuous casting of flat, thin products and, more particularly to an improvement in refractory plates consisting in inserting thereon ceramic inserts against wear.
From JP-A-7068352 is disclosed a ceramic plate which is mounted on water cooled metallic case in order to make thermal deformation and side wear small in a casting twin rolls apparatus.
Refractory plates for the lateral containment of molten metal, in an apparatus for the continuous casting of thin, flat products and having ceramic inserts thereon are already known.
Presently, such plates are produced according to a known technique including the following steps:
casting a material having a high content of SiC;
creating areas destined for the containment of the inserts;
firing at a high temperature (1250xc2x0 C.) of the plate thus obtained;
inserting, by means of a refractory cement having a high content of ZrO2 (or SiC), the inserts of composite ceramic material (constituted, for example, of BN/Sialon or BN/SiC/ZrO2);
casting a silica-alumina refractory material having a high content of ZrO2 on the preformed plate of the previous casting; and
firing at a low temperature (about 400xc2x0 C.) of the plate so constituted.
Such technique for the production of refractory plates having ceramic material inserts, shows the disadvantage of being time-consuming and awkward. Moreover, it has the inconvenience of not allowing the achievement of plates with a good planarity caused by the fact that the inserts have level differences each other, even if these have a reduced value.
Another drawback of such technique consists in that, owing to the dimensional contraction of the casting and of the cement employed for the fastening of said inserts on the casting, some times there occur fissures between the inserts and adjacent parts of the plate. These fissures lead to possible infiltrations of molten metal with the consequent build-up of solid metal on the surface of the same. Such solid metal becomes the nucleus for the growth of solid steel crusts that endanger the whole casting process.
Thus, the aim of the present invention is to provide improved refractory plates having inserts of ceramic material that solve the abovementioned problems of reduced planarity, of a time-consuming and difficult production of the same, and of the generation of fissures in the areas between inserts and refractory castings.
Another aim of the present invention is to provide low-cost improved refractory plates with ceramic material inserts.
A further object of the present invention is to provide a process for the production of such improved refractory plates.
Therefore, according to the present invention, there are provided improved refractory plates for the lateral containment in apparatuses for the continuous casting, characterised in that they comprise:
at least one insert of a ceramic material;
a plurality of joints for the thermal expansion, the joints being constituted of a ceramic fibres material;
a first casting of a silica-alumina material containing ceramic fibres;
a second casting of a silica-alumina material having a high content of ZrO2; and
a third casting of a SiC-based thixotropic material;
the arrangement being such that said at least a ceramic insert is arranged on said third casting of a thixotropic material so that it assumes a substantially triangular shape, said first silica-alumina caseting based on a ceramic fibres material is arranged on said third casting at the exterior area with respect to said at least a ceramic insert, said second casting of silica-alumina material having a high contents of ZrO2 is arranged on said third casting of a thixotropic material substantially at the central portion of the plate defined by said at least a ceramic insert, and said plurality of joints in ceramic fibres for the thermal expansion is arranged at the contacting surfaces between said at least a ceramic insert and at least one of said first, second and third casting.
Moreover, the present invention provides a process for the production of refractory plates with ceramic inserts, characterised in that it comprises the following steps:
positioning and optimised coupling, by means of gluing, of at least one insert of ceramic material on the bottom of a mould;
positioning with an optimised coupling of a plurality of joints constituted in ceramic fibres, on the peripheral walls of said at least one insert of ceramic material;
carrying out of a first casting of a refractory material, substantially constituted of a mixture of silica-alumina and ceramic fibres, at the external part of said at least one ceramic insert;
carrying out of a second casting of a silica-alumina refractory material having a high contents of ZrO2, at the central part of the plate and internally bounded by said at least one ceramic insert;
carrying out of a third casting of a refractory thixotropic material with a high contents of SiC, above said at least one ceramic insert, with respect to said first casting of refractory material and said second casting of refractory material having a high contents of ZrO2;
removing the plate thus constituted from said mould, after a time of at least 24 hours;
thermal treating the plate at 110xc2x0 C. for a period of time of at least 24 hours; and
firing at low temperature said plate at least at 400xc2x0 C. for a time period of at least 1 hour.